Biomedical subjects

Romuald Lepers

Publications and source records attributed to Romuald Lepers.

14 recordsLinked to original sources

PubMed2007

Assessment of the reliability of central and peripheral fatigue after sustained maximal voluntary contraction of the quadriceps muscle.

The aim of the present study was to further confirm the validity of measurements for characterizing neuromuscular alterations by establishing their reliability both before and after fatigue. Thirteen men (28 +/- 5 years) volunteered to participate in two separate identical sessions requiring the performance of a sustained maximal voluntary contraction (MVC) with the quadriceps muscle for 2 min. MVC and transcutaneous electrical stimulations were used before and immediately after the fatiguing contraction to investigate the reliability of MVC torque, central activation, and peripheral variables (M-wave properties, peak twitch, peak doublet) within and between sessions. Based on previous and present results, we advise the use of (1) voluntary activation level with potentiated doublet as a reference to describe central fatigue, (2) electromyographic activity of vastus lateralis muscle as a surrogate for quadriceps for both voluntary and evoked contraction, and (3) potentiated peak doublet amplitude to investigate contractile fatigue. These findings can be useful in the choice of the parameters describing central and peripheral fatigue of the quadriceps muscle in future studies.

Action Potentials↗

PubMed2006

Differences in cardiorespiratory and neuromuscular responses between voluntary and stimulated contractions of the quadriceps femoris muscle.

The aim of this study was to compare respiratory gas exchange variables and muscle fatigue between equal-intensity (i.e., same force output) electrostimulated and voluntary contractions of the quadriceps muscle (46+/-10% of maximal voluntary force). Twelve healthy men served as volunteers. Oxygen consumption, ventilation and respiratory exchange ratio were recorded during the exercise bouts. Muscle fatigue was quantified as the exercise-induced reduction in maximal voluntary force. The average oxygen consumption (11+/-3 versus 8+/-2 mL min(-1)kg(-1)), ventilation (23+/-4 versus 16+/-2 L min(-1)) and respiratory exchange ratio (0.96+/-0.02 versus 0.85+/-0.01) were significantly higher during electrostimulation compared with voluntary exercise (P<0.05-0.001). Maximal voluntary force decreased significantly after electrostimulation (-21+/-10%; P<0.001), while no changes were observed following voluntary exercise. Electrostimulation-resistance exercise of the quadriceps muscle elicited greater cardiorespiratory demand and muscle fatigue compared with voluntary contractions of the same intensity. These findings probably reflect differences in the patterns of motor unit recruitment between stimulated and voluntary contractions, despite equal force productions.

Adult↗

PubMed2006

Muscle activation during cycling at different cadences: effect of maximal strength capacity.

The purpose of this study was to examine the influence of maximal strength capacity on muscle activation, during cycling, at three selected cadences: a low cadence (50 rpm), a high cadence (110 rpm) and the freely chosen cadence (FCC). Two groups of trained cyclists were selected on the basis of the different maximal isokinetic voluntary contraction values (MVCi) of their lower extremity muscles as follow: F(min) (lower MVCi group) and F(max) (higher MVCi group). All subjects performed three 4-min cycling exercises at a power output corresponding to 80% of the ventilatory threshold under the three cadences. Neuromuscular activity of vastus lateralis (VL), rectus femoris (RF) and biceps femoris (BF) was studied quantitatively (integrated electromyography, IEMG) and qualitatively (timing of muscle bursts during crank cycle). Cadence effects were observed on the EMG activity of VL muscle and on the burst onset of the BF, VL and RF muscles. A greater normalized EMG activity of VL muscle was observed for the F(min) group than the F(max) group at all cadences (respectively F(min) vs. F(max) at 50 rpm: 17+/-5% vs. 38+/-6%, FCC: 22+/-7% vs. 44+/-5% and 110 rpm: 21+/-6% vs. 45+/-6%). At FCC and 110 rpm, the burst onset of BF and RF muscles of the F(max) group started earlier in the crank cycle than the F(min) group These results indicate that in addition to the cadence, the maximal strength capacity influences the lower extremity muscular activity during cycling.

Adult↗

PubMed2006

Neuromuscular fatigue differs with biofeedback type when performing a submaximal contraction.

The aim of the study was to examine alterations in contractile and neural processes in response to an isometric fatiguing contraction performed with EMG feedback (constant-EMG task) when exerting 40% of maximal voluntary contraction (MVC) torque with the knee extensor muscles. A task with a torque feedback (constant-torque task) set at a similar intensity served as a reference task. Thirteen men (26+/-5 yr) attended two experimental sessions that were randomized across days. Endurance time was greater for the constant-EMG task compared with the constant-torque task (230+/-156 s vs. 101+/-32s, P<0.01). Average EMG activity for the knee extensor muscles increased from 33.5+/-4.5% to 54.7+/-21.7% MVC EMG during the constant-torque task (P<0.001), whereas the torque exerted during the constant-EMG task decreased from 42.8+/-3.0% to 17.9+/-5.6% MVC torque (P<0.001). Comparable reductions in knee extensors MVC (-15.7+/-8.7% for the constant-torque task vs. -17.5+/-9.8% for the constant-EMG task, P>0.05) and voluntary activation level were observed at exhaustion. In contrast, excitation-contraction coupling process, assessed with an electrically evoked twitch and doublet, was altered significantly more at the end of the constant-EMG task despite the absence of M-wave changes for both tasks. Present results suggest that prolonged contractions using EMG biofeedback should be used cautiously in rehabilitation programs.

Adult↗

PubMed2006

Synergists activation pattern of the quadriceps muscle differs when performing sustained isometric contractions with different EMG biofeedback.

The aims of the present study were to examine (1) endurance time and (2) activation pattern of vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) muscles during fatiguing isometric knee extensions performed with different EMG biofeedbacks. Thirteen men (27 +/- 5 year) volunteered to participate in three experimental sessions. Each session involved a submaximal isometric contraction held until failure at an EMG level corresponding to 40% maximal voluntary contraction torque (MVC), with visual EMG biofeedback provided for either (1) RF muscle (RF task), (2) VL and VM muscles (Vasti task) or (3) the sum of the VL, VM and RF muscles (Quadriceps task). EMG activity of VL, VM and RF muscles was recorded during each of the three tasks and further analyzed. Time to task failures and MVC loss (P < 0.001) after exercises were similar (P > 0.05) between the three sessions (182 s and approximately 28%, respectively) (P > 0.05). Moreover, the magnitude of central and peripheral fatigue was not different at failure of the three tasks. Activation pattern was similar for knee extensors at the beginning of each task (P > 0.05). However, RF EMG activity decreased (P < 0.05) during the Vasti and the Quadriceps tasks (from approximately 33 to approximately 25% maximal EMG), whereas vasti EMG activity remained constant during the RF task ( approximately 41% maximal EMG). These findings suggest that (1) task failure occurs when sustaining a submaximal level of EMG activity for as long as possible and (2) CNS is not able to differentiate descending drive to the different heads of the quadriceps at the beginning of a sustained contraction, despite a different activation pattern for the bi-articular RF muscle compared to the mono-articular vasti muscles during fatigue.

Adult↗

PubMed2006

Vitamin and mineral supplementation and neuromuscular recovery after a running race.

PURPOSE: This double-blind study investigated the effects of vitamin and mineral complex supplementation on the neuromuscular function of the knee-extensor muscles after a prolonged trail running race. METHODS: Twenty-two well-trained endurance runners took either placebo (Pl group) or vitamins and minerals (Vm group) for 21 d before the race and for 2 d after the race. Maximal voluntary contractions (MVC) and surface EMG activity of the vastus lateralis (VL) muscle were recorded before (pre) and 1 h (post), 24 h (post 24) and 48 h (post 48) after the race. Central activation ratio (CAR), neural (M-wave), and contractile (muscular twitch) properties of the quadriceps muscles were analyzed using electrical stimulation techniques. RESULTS: The knee-extensor MVC was significantly (P < 0.01) reduced after exercise for both groups (Vm: 36.5 +/- 3.0 %; Pl: 36.9 +/- 2.1%), but MVC recovery was greater for Vm than Pl after 48 h (11%, P < 0.05). The reduced MVC after exercise was associated with a significant reduction in maximal EMG normalized to the M-wave in VL muscle and in CAR for both groups. Characteristics of the muscular twitch were not significantly altered for either groups, whereas M-wave duration increased significantly (P < 0.05) after exercise. CONCLUSIONS: The reduction of MVC immediately after the race appeared to result from peripheral mechanisms such as a failure in muscle membrane excitation and, to a lesser extent, from reduced central activation. The cause of the depressed MVC 24 h after the race seemed to be located within the muscle itself. A dietary supplementation of a vitamin and mineral complex does not attenuate the loss of contractile function immediately after the running exercise, and it may accelerate the recovery of maximal force capacity.

Adult↗

PubMed2005

Changes in neuromuscular function after tasks involving control of EMG versus torque feedback of the same duration.

This study was designed to compare alterations in neuromuscular function after two tasks of similar duration involving the control of (1) torque level fixed at 40% maximal voluntary contraction (MVC) torque (torque task) and (2) EMG level when exerting 40% MVC torque on the knee extensor muscles. Ten healthy subjects volunteered to participate in two testing sessions separated by approximately 2 h. Contraction duration for the EMG task was fixed for each subject to the time to task failure of the torque task (104+/-20s). MVC, maximal voluntary activation level, muscle compound action potential (M-wave), peak twitch and potentiated peak doublet were assessed before and immediately after each task using electrical stimulation of the femoral nerve. Average EMG activity of quadriceps muscle increased (p<0.01) during the torque task from 27.7+/-5.4% to 46.2+/-19.3% maximal EMG, whereas torque decreased during the EMG task from 41.5+/-2.9% to 28.9+/-3.8% MVC torque. Alterations in MVC torque (p<0.01) and maximal voluntary activation level (p<0.05) were comparable at termination of the two tasks. Rate of perceived exertion was greater (p<0.05) at the end of the torque task compared to the EMG task. Despite the absence of change in the M-wave for either task, potentiated peak doublet was altered after the torque task (-18+/-14%, p<0.01), whereas there was no change after the EMG task (p>0.05). The absence of peripheral failure at the end of the EMG task could be attributed to (1) a lower intramuscular pressure allowing a lesser accumulation of metabolites and (2) a slower rate of PCr hydrolysis compared to the torque task.

Adult↗

PubMed2004

Twitch potentiation is greater after a fatiguing submaximal isometric contraction performed at short vs. long quadriceps muscle length.

Endurance time of a submaximal sustained contraction is longer when the muscle is fatigued in a shortened position. The aim of the present study was to compare central and peripheral mechanisms of fatigue after an isometric contraction of the knee extensor muscles performed at 20% maximal voluntary contraction (MVC) at two knee angles (35 degrees , short length vs. 75 degrees , long length; 0 degrees = full extension) until exhaustion. Eleven men (24 +/- 4 yr) attended two experimental randomized sessions. Endurance time was greater at 35 degrees compared with 75 degrees (974 +/- 457 vs. 398 +/- 144 s; P < 0.001) despite a similar reduction in knee extensor MVC (-28.4 +/- 16.0%, P < 0.001 vs. -27.6 +/- 18.8%, P < 0.001, respectively). Voluntary activation level was similarly depressed after the fatiguing contraction performed at the two muscle lengths (-19 +/- 16.7% at 35 degrees , P < 0.01 vs. -13.7 +/- 14.5% at 75 degrees , P < 0.01). After the fatiguing contraction, peak twitch potentiation was observed only at the short length (+31.8 +/- 17.6% at 35 degrees , P < 0.01 vs. +6.4 +/- 21.3% at 75 degrees , P > 0.05), whereas M-wave properties were similarly altered for the two angles. These results suggest that 1) central fatigue at task failure for a sustained isometric contraction was not dependent on the muscle length, and 2) the longer endurance time of a sustained isometric contraction performed at a shortened length is related to potentiation. It is suggested that the greater endurance time of a sustained isometric contraction observed at 35 degrees is related to the occurrence of potentiation at this short length, because central fatigue is similar at task failure for both tasks.

Adaptation, Physiological↗

PubMed2004

Time course of neuromuscular alterations during a prolonged running exercise.

PURPOSE: This study investigated the time course of contractile and neural alterations of knee extensor (KE) muscles during a long-duration running exercise. METHODS: Nine well-trained triathletes and endurance runners sustained 55% of their maximal aerobic velocity (MAV) on a motorized treadmill for a period of 5 h. Maximal voluntary contraction (MVC), maximal voluntary activation level (%VA), and electrically evoked contractions (single and tetanic stimulations) of KE muscles were evaluated before, after each hour of exercise during short (10 min) interruptions, and at the end of the 5-h period. Oxygen uptake was also measured at regular intervals during the exercise. RESULTS: Reductions of MVC and %VA were significant after the 4th hour of exercise and reached -28% (P < 0.001) and -16% (P < 0.01) respectively at the end of the exercise. The reduction in MVC was highly correlated with the decline of %VA (r = 0.98, P < 0.001). M-wave was also altered after the fourth hour of exercise (P < 0.05) in both vastus lateralis and rectus femoris muscles. Peak twitch was potentiated at the end of the exercise (+18%, P = 0.01); 20- and 80-Hz maximal tetanic forces were not altered by the exercise. Oxygen uptake increased linearly during the running period (+18% at 5 h, P < 0.001). CONCLUSION: These findings suggest that KE maximal voluntary force generating capability is depressed in the final stages of a 5-h running exercise. Central activation failure and alterations in muscle action potential transmission were important mechanisms contributing to the impairment of the neuromuscular function during prolonged running.

Adult↗

PubMed2004

Alterations of neuromuscular function after prolonged running, cycling and skiing exercises.

It is well known that impairment of performance resulting from muscle fatigue differs according to the types of contraction involved, the muscular groups tested and the exercise duration/intensity. Depending on these variables, strength loss with fatigue can originate from several sites from the motor cortex through to contractile elements. This has been termed 'task dependency of muscle fatigue'. Only recently have studies focused on the origin of muscle fatigue after prolonged exercise lasting 30 minutes to several hours. Central fatigue has been shown to contribute to muscle fatigue during long-distance running by using different methods such as the twitch interpolation technique, the ratio of the electromyogram (EMG) signal during maximal voluntary contraction normalised to the M-wave amplitude or the comparison of the forces achieved with voluntary- and electrically-evoked contractions. Some central activation deficit has also been observed for knee extensor muscles in cycling but central fatigue after activities inducing low muscular damage was attenuated compared with running. While supraspinal fatigue cannot be ruled out, it can be suggested that spinal adaptation, such as inhibition from type III and IV group afferents or disfacilitation from muscle spindles, contributes to the reduced neural drive after prolonged exercise. It has been shown that after a 30 km run, individuals with the greatest knee extensor muscle strength loss experienced a significant activation deficit. However, central fatigue alone cannot explain the entire strength loss after prolonged exercise. Alterations of neuromuscular propagation, excitation-contraction coupling failure and modifications of the intrinsic capability of force production may also be involved. Electrically-evoked contractions and associated EMG can help to characterise peripheral fatigue. The purpose of this review is to further examine the central and peripheral mechanisms contributing to strength loss after prolonged running, cycling and skiing exercises.

Bicycling↗

PubMed2003

Activation among the elbow flexor muscles differs when maintaining arm position during a fatiguing contraction.

Twenty-four men (n = 11) and women (n = 13) supported an inertial load equivalent to 20% of the maximum voluntary contraction force with the elbow flexor muscles for as long as possible while maintaining a constant elbow angle at 90 degrees. Endurance time did not differ on the three occasions that the task was performed (320 +/- 149 s; P > 0.05), and there was no difference between women (360 +/- 168 s) and men (273 +/- 108 s; P = 0.11). The rate of increase in average electromyogram (EMG) for the elbow flexor muscles was similar across sessions (P > 0.05). However, average EMG during the fatiguing task increased for the long head of biceps brachii, brachioradialis, and brachialis (P < 0.05) but not for the short head of biceps brachii. Furthermore, the average EMG for the brachialis was greater at the start and end of the contraction compared with the other elbow flexor muscles. The rate of bursts in EMG activity increased during the fatiguing contraction and was greater in brachialis (1.0 +/- 0.2 bursts/min) compared with the other elbow flexor muscles (0.5 +/- 0.1 bursts/min). The changes in the standard deviation of acceleration, mean arterial pressure, and heart rate during the fatiguing contractions were similar across sessions. These findings indicate that the EMG activity, which reflects the net excitatory and inhibitory input received by the motoneurons in the spinal cord, was not adaptable over repeat sessions for the maintain-position task. Furthermore, these results contrast those from a previous study (Hunter SK and Enoka RM. J Appl Physiol 94: 108-118, 2003) when the goal of the isometric contraction was to maintain a constant force. These results, from a series of studies on the elbow flexor muscles, indicate that the type of load supported during the fatiguing contraction influences the extent to which endurance time can change with repeat performances of the task.

Acceleration↗

PubMed2003

Quadriceps femoris torque and EMG activity in seated versus supine position.

PURPOSE: To compare voluntary and electrically evoked knee extensor torque, surface electromyography (EMG), and activation level obtained under seated versus supine position, i.e., with shortened versus lengthened rectus femoris (RF) muscle. METHODS: The knee extensor torque obtained during maximal voluntary contractions (MVC) and after single and paired stimuli applied at rest was measured under seated (i.e., 90 degrees hip angle) and supine (i.e., 180 degrees hip angle) conditions. The associated EMG activity from biarticular RF and monoarticular vasti was also recorded, and activation level was estimated by means of the twitch interpolation technique. RESULTS: Knee extensor MVC was 10.6% higher in the seated compared with the supine position (P < 0.01). EMG normalized to the M-wave amplitude for respective muscles and for respective positions was significantly lower under supine conditions, and deficits averaged approximately 20% for vasti and approximately 41% for RF (P < 0.05). Similarly, activation level estimated in the supine position was approximately 4% lower than the seated counterpart. Relative MVC losses observed supinely were significantly correlated with the corresponding activation level deficits (P < 0.05). On the other hand, both single and paired stimuli resulted in higher torque amplitudes in the supine with respect to the seated position, and mean differences were comprised between 10% (single twitch, P < 0.05) and 20% (potentiated doublet, P < 0.001). RF M-wave amplitude recorded supinely was 19% higher than the seated counterpart (P < 0.01). CONCLUSION: The higher neural activation observed for the knee extensor muscles in the seated versus supine position, likely attributable to improved motor unit recruitment, may reflect a neurophysiological mechanism partly compensating the neuromuscular transmission-propagation impairment and/or mechanical disadvantage of shortened RF muscle.

Adult↗

PubMed2003

Neuromuscular differences between endurance-trained, power-trained, and sedentary subjects.

This study tested the hypothesis that neuromuscular characteristics of plantar flexor (PF) and knee extensor (KE) muscles explain differences of both performance in vertical jump and maximal voluntary isometric contraction (MVC) between endurance-trained (END, n = 9), power-trained (POW, n = 8), and sedentary subjects (SED, n = 8). Evoked twitch characteristics of PF and KE were measured. MVC, maximal voluntary activation (%VA) of KE, and performance in vertical jump were also measured. POW have higher maximal rate of twitch force development (MRFD) than SED and END for both PF (p < 0.05) and KE (p < 0.01); %VA and MVC were higher for POW and END than SED (p < 0.01). Higher performances were measured in vertical jump for POW compared with END and SED. Significant relationships were found between the squat jump performance and MRFD for both KE and PF (R = 0.71, p < 0.0001 and R = 0.55, p < 0.01, respectively). These findings show that low MRFD on lower limbs extensors does not limit expression of MVC on subjects with high levels of activation, whereas intrinsic muscular qualities have a direct influence on performance during the vertical jump.

Adult↗

PubMed2002

Neuromuscular fatigue during a long-duration cycling exercise.

The effects of prolonged cycling on neuromuscular parameters were studied in nine endurance-trained subjects during a 5-h exercise sustained at 55% of the maximal aerobic power. Torque during maximal voluntary contraction (MVC) of the quadriceps muscle decreased progressively throughout the exercise (P < 0.01) and was 18% less at the end of exercise compared with the preexercise value. Peak twitch torque, contraction time, and total area of mechanical response decreased significantly (P < 0.05) after the first hour of exercise. In contrast, changes in M-wave characteristics were significant only after the fourth hour of the exercise. Significant reductions (P < 0.05) in electromyographic activity normalized to the M wave occurred after the first hour for the vastus lateralis muscle but only at the end of the exercise for the vastus medialis muscle. Muscle activation level, assessed by the twitch interpolation technique, decreased by 8% (P < 0.05) at the end of the exercise. The results suggest that the time course is such that the contractile properties are significantly altered after the first hour, whereas excitability and central drive are more impaired toward the latter stages of the 5-h cycling exercise.

Adult↗